The printed circuitry development has been one of the important elements in the tremendous growth of the electronic industry. As it is well known, printed circuitry may be obtained by a number of processes, the merits of which are disclosed in patents such as U.S. Pat. No. 3,562,005. In this patent the various prior art processes for the depositing of metal on a substrate have been evaluated with the shortcomings and advantages of each process being discussed. Moreover, in this patent the various means how electroless plating has been accomplished have been described. Still further, in U.S. Pat. No. 3,562,005, the "photo promoter" process has also been disclosed.
The further deposition of metal on the "photo promotor" deactivated surface is described as electroless plating in this patent as well as in a number of references. A further reference which is illustrative of the various processes is J. J. Webicki, Practical Electroless and Immersion Plating, Plating, volume 58 (8), pp 763-767 (1971).
Inasmuch as the electroless deposition takes place by the chemical reduction of a metal salt on the surface of a substrate, either on a metal or on a non-conductive substrate surface (which has been "catalyzed" to receive the metal sought to be deposited), various complexing agents have been proposed for forming a catalytic metal on the surface of a substrate. The search is still being continued for more easily controllable catalytic agents or catalytic agents which are capable of making an enhanced and stronger bond between the substrate, often an organic substrate, and the metal sought to be deposited.
Still further, a continuous search has been for complexes which would (a) lower the activation energy of the reactive methal or (b) increasee the solubility of the metal salt or the metal salts in the electroless bath. With respect to to the latter, the increased solubility provides an increase in concentration of the reactants and hence increases the reaction rate during the reduction reaction. With reference to approach (a) above, a lowering of the activation energy also implies an increase in reaction rate.
With respect to electroless plating, a number of drawbacks have become evident when using the prior art processes. For example, the pH and the temperature affect the reaction and must be carefully controlled because the electroless baths are usually operated at conditions at which the baths are likely to undergo homogeneous decomposition. Although buffering agents are used, sometimes to attempt the maintenance of the pH within as narrow range as possible, buffering agents introduce additional elements in the bath detrimental to the deposition process. Still further, wetting agents have been proposed as avoiding some of the problems and stabilizers have been employed which are alleged to improve the operation of the baths. Nevertheless, these agents and stabilizers again add further contaminant sources and thus potential contaminants are introduced in the bath. Hence, the baths become further hard to control because of the cumulative effect of the additives.
With respect to the prior art processes, many of the aqueous electroless and more particularly gold electroless plating solutions contain hypophosphites as the reducing agents. For example, U.S. Pat. Nos. 1,207,218 and 2,976,181 disclose bath containing hypophosphites as the reducing agent, but these baths are limited by reaction products generated in the bath. For example, phosphides and phosphates build up in the bath. Auto degeneration of reducing agents also sets in. These shortcomings of the prior art have been pointed out in other patents such as U.S. Pat. No. 3,032,436. In the last mentioned patent, it has been proposed to use hydrazine hydrate as the reducing agent to avoid the problems associated with prior art baths. However, hydrazine is a thermodynamically unstable material and is prone to spontaneous decomposition. In order to operate safely and obtain optimum limits when using bath with hydrazine as a reducing agent, the pH of the solution must be continually monitored and the reducing agent must be added in small quantities.
Although it is known that palladium salts may act as reducing agents, such as disclosed in U.S. Pat. No. 3,396,042, the pH of the solution must be carefully controlled between 8 to 11 and preferably on the high pH value side of this range. If the pH of the solution is not carefully controlled, codeposition of palladium is a continuous problem. Consequently, gold deposits are obtained with lower purity having undesirable characteristics making the gold deposits unacceptable for a number of different uses.
Additional patents have described the use of gold cyanide or potassium gold cyanide as the source of gold in an electroless bath. For example, U.S. Pat. No. 3,506,462 describes these cyanides but an extreme care must be exercised so that no acid is introduced in the bath which would cause the extreme toxicity associated with hydrogen cyanide. Still further, in many electronic applcations, especially for semi-conductor materials, the absence of sodium or potassium which is detrimental is very important. Hence, the plating of these materials with gold from potassium gold cyanide or utilizing an alkaline metal as the reducing agent or complexing agent is not acceptable. (Cf. U.S. Pat. No. 3,300,328).
For the reasons previously mentioned and for the further reasons that the electroless plating bath is operated under conditions such that spontaneous decomposition of the bath is likely to occur, the commercial usefulness of the baths have been limited or a need for considerable improvement manifested. Still further, the gold electroless baths have not been used because these baths have not been capable of regeneration once a small fraction of the gold has been removed, (i.e. 25%) and at these conditions the bath has stopped plating.
Although some phosphorous containing complexes and their use in immersion plating and thermal decomposition processes has been described in U.S. Pat. Nos. 3,438,805 and 3,492,151, the electroless process has not been described in these patents.